run-tests: forward Python USER_BASE from site (issue5425)

We do this so that any linters installed via pip install --user don't
break. See https://docs.python.org/2/library/site.html#site.USER_BASE
for a description of what this nonsense is all about.

An alternative would be to not set HOME, but that'll cause other
problems (see issue2707), or to forward every single path entry from
sys.path in PYTHONPATH (which seems sketchy in its own way).

tests/run-tests.py

@@ -58,6 +58,11 b' import signal'
 import socket
 import subprocess
 import sys
+try:
+    import sysconfig
+except ImportError:
+    # sysconfig doesn't exist in Python 2.6
+    sysconfig = None
 import tempfile
 import threading
 import time
@@ -818,6 +823,8 b' class Test(unittest.TestCase):'
             offset = '' if i == 0 else '%s' % i
             env["HGPORT%s" % offset] = '%s' % (self._startport + i)
         env = os.environ.copy()
+        if sysconfig is not None:
+            env['PYTHONUSERBASE'] = sysconfig.get_config_var('userbase')
         env['TESTTMP'] = self._testtmp
         env['HOME'] = self._testtmp
         # This number should match portneeded in _getport